Conventionally, train carriages are ventilated by intake and exhaust ducts with respective electric blowers by which air is collected from outside a carriage passed through the interior space of the carriage, and exhausted to the exterior. During operation the exterior air pressure varies substantially. In particular it fluctuates when two trains pass one another, when a train enters or leaves a tunnel, and most particularly when two trains pass one another in a tunnel. The magnitude of the fluctuation varies approximately in proportion to the square of the train velocity, assuming closely-matched sectional areas for the trains and the tunnel.